Plasma torches are important for a different of types of elemental analysis. Inductively Coupled Plasma (ICP) torches torches are an integral part of ICP analytical systems that perform mass spectrometry (MS) or optical emission spectroscopy (OES).
ICP torches commonly include three concentric tubes including an inner, sample tube for delivering an aerosol sample to the plasma within the torch; a middle, plasma tube (which is frequently flared) for delivering the plasma gas; and an outer, coolant tube for delivering a coolant gas. The outer tube of the ICP torch has a plasma surrounding portion that distally extends beyond the distal tips of the respective middle and inner tubes. Gas flow through the sample and plasma tubes are axial, but the flow through the coolant tube follows a helical path along the inside annulus of the tube.
The three tubes are typically in concentric alignment over a length of 5-20 centimeters, and the required tolerance is very fine. As a result ICP torches have conventionally been formed using three tubes fused at one end to fix the tolerances required for consistent operation.
In use, the ICP torch is mounted in a box/cavity within the spectroscopy equipment. The box includes an RF coil, which surrounds the plasma surrounding portion of the outer tube, to supply sufficient energy to sustain the plasma.
To ignite (seed) the plasma a Tesla coil provides spark at a side of the coolant (outer) tube, at a location that is more proximally located (with respect to the base of the torch) than the plasma surrounding portion of the outer tube. At that location, a cross section through the torch includes both the outer and middle tubes (and generally the inner tube as well). The spark is transmitted through the outer tube, from where electrons from the spark travel helically and distally along the tube to seed the generation of the plasma.
A torch in which the tubes are fused is difficult to clean or repair, for example when the sample tube is contaminated or when the coolant tube undergoes melt-down. Therefore demountable torches have been designed to enable at least the outer tube to be removably mounted to a part of the torch assembly.
U.S. Pat. No. 7,847,210 B2 (Brezni et al.) dated 7 Dec. 2010, the entirety of which is incorporate herein by reference, describes torches in which the outer tube has a tubular body and a mounting feature projecting from the tubular body for controlling alignment of the tubular body to a base portion of the torch assembly. In some embodiments, the mounting feature also releasably secures the tubular body to the base portion of the torch assembly, while in other embodiments a further part is used to releasably hold the mounting feature to the base portion to thereby secure the tubular body in the torch assembly. In various embodiments the mounting feature includes a tapered surface to control the alignment, the tapered surface being on a collar that is integral with the outer tube at a proximal end of the outer tube.
Standard ICP torches have transparent outer, middle and inner tubes, each comprised of quartz. The transparency of the outer tube allows light emitted from the plasma to be detected by a light sensor in the box. If the light level that is incident upon the sensor is below a certain threshold, the spectrometer will determine that there is no plasma present. Thus if a plasma is expected but is not present, appropriate action may be taken by the spectroscopy equipment, for example the equipment may notify or alert an operator. The transparency of the outer tube also enables the plasma within the torch to be seen by the operator from a side-on view of the torch, through a viewing window in the box that houses the torch. Thus the operator may visually verify whether the plasma is present and/or observe the plasma for diagnostic or other purposes.
However, the plasma can reach temperatures in excess of 6000° C., which is well above the melting point of quartz, which is in the order of 1700° C. (more specifically about 1670° C.). To provide some protection against damage to the tubes (the outer and middle tubes in particular), a coolant gas (typically Argon) is supplied via the outer tube, and over the middle tube, to cool the outer tube. However, a significant flow rate of gas is required to achieve sufficient cooling and this can be financially costly. Even still, the such quartz tubes may need to be replaced as often as once a week, depending on the specific application and amount of use.
To improve the longevity of ICP torches and/or to reduce the amount of required coolant gas, the tubes of the ICP torch are made to have a higher melting point than that of quartz. However, such materials, such as many ceramics, may be opaque, thus hiding the plasma from the operator and from the light sensor in the torch box.
Also, in the case of quartz tubes, the spark transmits relatively well through the tubes, but some ceramic tubes have a higher impedance against such transmission. This can result in the ignition of the plasma being less consistent in the case of such ceramic tubes. To reduce the outer tube's impedance to spark transmission, some of such ceramic outer tubes include a circular hole in the ceramic wall of the tube, at the termination of the tesla coil. To avoid leakage of coolant gas though the hole, a copper or other low impedance material is patched over the hole. However, this adds a part to the torch assembly and may be considered visually unsatisfactory for some users.
The present invention provides an ICP plasma torch or a component for such a torch that addresses at least one of these or other problems of the prior art.
Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.